1. Field of the Invention
The invention relates generally to marine seismic exploration and surveying and, more particularly, relates to improved receiver systems for use in such marine seismic exploration.
2. Description of Related Art
Marine seismic surveying is generally conducted using a seismic survey vessel equipped with an energy source and seismic detector for taking seismic profiles of an underwater land configuration. The source is typically an air gun or explosive device which imparts an acoustic wave to the water, creating a wavefield which travels coherently into the underlying earth. As the wavefield propagates through the underlying earth, it strikes interfaces between formations commonly referred to as strata, and reflects back through the earth and water toward a receiver. The receiver converts the reflected signals to electrical signals which are recorded and may then be analyzed to determine geological details of formations beneath the sea bottom.
Among the seismic detectors typically used are geophones, which are devices sensitive to particle motion, and hydrophones, devices responsive to changes in pressure. Geophones detect particle motion and changes in velocity or acceleration of particle motion which result from vibrations in the external environment and are inherently directional in nature. Hydrophones detect changes in pressure, generally without regard to direction.
A frequently employed type of detector arrangement consists of a series of hydrophone/geophone pairs which are attached to or incorporated within an elongated sensor cable towed by the seismic survey vessel and which is then positioned on the sea bottom. The energy source of the towing survey vessel emits seismic signals from predetermined locations usually by means of an airgun or explosive device. The seismic signals generated by the source are reflected back from strata beneath the ocean bottom. The reflected signals are then recorded by the hydrophone/geophone pairs. In some instances, the sensor cable is towed behind the vessel during recording rather than being deployed upon the ocean bottom.
The hydrophones are employed as receivers for reflected seismic pressure waves. The geophones record fluid particle motion changes induced by the seismic signals. The simultaneous recording of particle motion and pressure is a well known method for improving the signal-to-noise ratio in seismic recording.
A long standing problem in marine surveying of this type has been the presence of "ghost" wavefields which may obscure the actual subterranean wavefield reflections. Ghost wavefields result from re-reflection of the upwardly moving reflected wavefield downwards from the water's surface. Ghost signals pose a problem in the recording of seismic data by obscuring data in some frequencies while amplifying data in others.
The most common type of hydrophone includes a piezoelectric element which converts physical pressure signals into electrical signals. Conventional geophones are typically constructed of a coil of wire which is positioned within a magnetic field and contained within a housing. The coil is suspended by springs and tends to remain fixed in space due to its mass, while the housing and magnetic structure move in response to motion within the environment, such as that induced by a seismic source. The relative movement of the wire coil within the magnetic field induces an electrical voltage across the coil, approximating motion within the environment. A damping resistor is typically connected across the coil to create a loop and thereby permit current to flow through the coil in relation to the relative acceleration of the coil with respect to the magnetic field. The current acts to damp the coil's motion by means of magnetic forces which act upon the electrified coil. This damping facilitates a more accurate representation of seismic signals detected by the device.
Maintenance of geophone-style devices in a proper orientation is necessary for a seismic survey to be relatively accurate. This may be difficult as the cable may end up rotated about its own longitudinal axis as it reaches the sea floor. It is also difficult to economically anchor the cable properly to the sea floor. As a result, the associated geophone may become horizontally disoriented and be unable to accurately determine changes in the vertical component of acceleration.
A number of devices have been developed in an attempt to meet this need. The incorporation of gimbals into geophones has been proposed. However, in practice these devices have not been generally successful in correcting geophones misoriented beyond about 20.degree. or more. In addition, gimbaled geophones are costly and complex to manufacture. The complexity of the unit tends to make it prone to leakage and mechanical damage as well as difficult to repair. Fluid-filled self-orienting vertically sensitive accelerometers have also been developed but have not found widespread acceptance.
A number of devices and methods have also been proposed for elimination or minimization of the "ghost" wavefield signals from the signals which are intended to be collected by the sensors. The combination of a vertical component accelerometer with a hydrophone, for instance, has been proposed for cancelling surface-reflected noise in marine seismic operations. The success of these devices, in dealing with the problem, however, is not clear.
A need exists for a device which addresses the problems of ghost wavefields and geophone misorientation.